1. Field of the Invention
The present invention relates to a comparator with a latching function that determines a level of a differential signal.
2. Description of the Related Art
Differential signals are used for transmitting a signal between semiconductor devices at a high speed. A test apparatus for testing a semiconductor device with a differential interface is provided with a comparator (timing comparator) that receives a differential signal outputted from the semiconductor device to be tested (hereinafter, referred to as a DUT: Device Under Test) and that performs a comparison/determination of a value of the signal at a certain timing and latches a comparison/determination result. The test apparatus determines whether the DUT is good or not, specifies a defective portion, or evaluates its performance, based on an output signal from the timing comparator.
The data transmission rate between semiconductor devices keeps on increasing in recent years, and the response speed needed for a test apparatus is increasing in accordance with it. Accordingly, a timing comparator is needed to have a higher-speed response with higher gain than before.
A comparator with a latching function is disclosed in FIG. 1 of Patent Document 1. The comparator comprises a first-stage differential amplifier and a latch circuit latching an output from the amplifier. With the structure, there is a problem that the range of an input voltage may be narrower as compared with that of a power supply voltage of a differential amplifier. When using a Rail-to-Rail amplifier performing in the full range between the power supply voltage and the ground voltage to deal with the problem, an operation speed is decreased in exchange for that. In addition, when a potential difference between the two input signals is small, there is a possibility that a circuit is unstable to cause an oscillation. When hysteresis is supplied thereto to deal with that, a dead-band occurs in exchange for that, causing the sensitivity to be decreased.
(Patent Document 1) U.S. Pat. No. 6,211,705B1, Description
(Patent Document 2) U.S. Pat. No. 4,558,241, Description
(Patent Document 3) U.S. Pat. No. 4,608,670, Description
(Patent Document 4) U.S. Pat. No. 4,461,965, Description
(Patent Document 5) U.S. Pat. No. 4,485,317, Description